The direct current (DC) characteristics of metal oxide semiconductor field effect transistors (MOSFETs) are routinely used to project alternating current (AC) performance of complementary metal oxide semiconductor (CMOS) technology. A simple metric relates the high speed gate delay to the measured DC current of the field effect transistors (FETs) as delay, D˜C*VDD/Id, where C is the total switching capacitance, VDD is the power supply voltage and Id is a measure of the positive channel field effect transistor (PFET) and negative channel field effect transistor (NFET) DC currents. Id may be measured at gate and drain voltages equal to VDD (Ion), at any other gate or drain voltage or may be averaged over different points in the drain current/drain voltage (IDS-VDS) curves suitable for a certain gate type. This metric has proven very useful in bulk Si technologies.
In Silicon-On-Insulator (SOI) CMOS technology, the Id from DC measurements is intrinsically different from the equivalent Id under high speed switching conditions. This difference arises from 1) self-heating effect in the body of the FET, raising its temperature and hence reducing Id under static DC measurements and 2) floating-body-voltage which modulates the threshold voltage; Vt and Id are different under DC and AC conditions. Measurement of FET currents with pulse inputs is made with specialized high frequency equipment, the complexity of which precludes it from being employed routinely in a manufacturing line. The current practice is to use the DC measured values with an estimated correction factor.
AC characteristics of MOSFETs are measured on the bench using high frequency probes and equipment. The measurements are time consuming and are not carried out routinely.
There is a need in the art for a method to directly measure both DC and AC (at frequencies in the GHz range) characteristics of the same MOSFETs using only DC inputs and outputs. There is a corresponding need in the art for a method to directly measure both AC and DC transfer characteristics of the same CMOS logic gates using only DC inputs and outputs.